The present invention relates generally to a printhead for ink-jet printers and, more particularly, to techniques for improving flatness and lamination quality in an ink-jet printhead.
The art of ink-jet printing is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with ink-jet technology for producing printed media.
Generally an ink-jet image is formed when a precise pattern of dots is ejected from a printhead onto a printing medium. Typically, an ink-jet printhead is supported on a movable carriage that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed.
A typical Hewlett-Packard ink-jet printhead includes an array of precisely formed nozzles in an orifice plate that is attached to a thin film substrate that implements ink firing heater resistors and apparatus for enabling the resistors. The thin film substrate is generally comprised of several thin layers of insulating, conducting or semiconductor material that are deposited successively on a supporting substrate in precise patterns to form, collectively, all or part of an integrated circuit. Deposition can be performed by mechanical, chemical or by vacuum evaporation methods.
In the printhead, an ink barrier layer defines ink channels including ink chambers disposed over associated ink firing resistors, and the nozzles in the orifice plate are aligned with associated ink chambers. Ink drop generator regions are formed by the ink chambers and portions of the thin film substrate and of the orifice plate that are adjacent the ink chambers.
The thin film substrate or die is typically comprised of a layer such as silicon on which are formed various thin film layers that form thin film ink firing resistors, apparatus for enabling the resistors, and interconnections to bonding pads that are provided for external electrical connections to the printhead. The thin film substrate more particularly includes a top thin film layer of tantalum disposed over the resistors as a thermomechanical passivation layer.
An example of the physical arrangement of the orifice plate, ink barrier layer, and thin film substrate is illustrated at page 44 of the Hewlett-Packard Journal of February, 1994. Further examples of ink-jet printheads are set forth in commonly assigned U.S. Pat. Nos. 4,719,577; 5,278,584 and 5,517,346, each of which is incorporated herein by reference.
Generally, circuit functionality determines thin film artwork. In this regard, differences in substrate thickness in functional, and in some cases nonfunctional, printhead regions, can result in structural failures of the printhead. For example, it is known that thin film topography can have a significant impact on the micro level. For example, in a typical stack, a tantalum layer is 6000 Angstrom units thick while a gold layer is 11000 Angstrom units thick and a metal-2 stack of tantalum/aluminum and aluminum has a thickness of 6000 Angstrom units. In some cases, thickness differences can be more pronounced where, for example, a metal-3 stack is modified. As used herein, the term "metal-2" refers to a composite comprising a thin film stack of tantalum-aluminum and aluminum. "Metal-3" refers to a composite of gold/tantalum.
Problems sometimes occur in conventional ink-jet printheads with respect to: a) barrier to orifice lamination and b) control of nozzle camber angle (NCA). With respect to lamination quality, for example, IJ5000 is a specially developed photoimageable dry film polymer that is used to define ink channels on the printhead. During the assembly process, this dry film is laminated onto the wafer where thin film topography is transmitted through the lamination thickness. In a subsequent assembly step, the polyimide orifice layer, flexible material sometimes 50 microns thick, is laminated to a singulated die.
This final lamination process, known as "staking", can result in printhead defects that are related to the thin film topography. These defects may be found in several printhead regions and, for example, may be evidenced by lack of corner lamination ("corner lift") or by the presence of bubbles between KaptonTm and barrier along the Ta/Au power trace boundary ("string bubbles").
With respect to the second condition, NCA is the relative angle between the orifice member in the nozzle region and center region (where the topography is assumed to be flat). Desirably, the printhead is coplanar with respect to the media being imprinted since any deviation from assumed coplanarity would produce dot placement error. Under ideal conditions, NCA is at or near 0 degrees. However, topographical considerations can affect NCA, generally tending to increase it. Thus, with respect to conventional ink-jet printheads, a need exists for controlling NCA, generally to reduce it. It is recognized however that under certain circumstances, there may be a need to increase NCA.
In view of the foregoing, a need exists for a technique for improving ink-jet printhead lamination quality to reduce substantially a likelihood of delamination between barrier and substrate. Desirably, the technique would include means of controlling NCA, to reduce it in the majority of cases but having a capability for increasing NCA if necessary.